Replace s3://test_bucket by s3://test-bucket in Riak CS docs

"test_bucket" doesn't seem to be a valid bucket name for Riak CS. s3cmd
throws the following error message:

  ERROR: S3 error: 400 (InvalidBucketName): The specified bucket is not
  valid
Merge pull request #268 from jbbarth/fix-invalid-bucket-name-in-riak-cs-doc

Replace s3://test_bucket by s3://test-bucket in Riak CS docs